Photodetectors made of mercury cadmium telluride semiconductor material are disclosed in U. S. Pat. No. 3,949,223, issued Apr. 6, 1976, and entitled, "Monolithic Photoconductive Detector Array."
As the '223 patent discloses, when radiation of the proper energy falls upon a semiconductor, its conductivity increases. Energy supplied to the semiconductor causes covalent bonds to break, and electron/hole pairs (also called majority/minority carriers) in excess of those generated thermally are created. These increased current carriers increase the conductivity of the material. This so-called "photoconductive effect" in semiconductor materials is utilized in photodetectors.
A photodetector can be, for example, a bar of semiconductor material having electrical contacts at its ends. In one form, the photodetector is connected in series with a direct current voltage source and a load resistor. The change in conductivity of the detector in response to incident radiation is sensed in one of two ways. If the resistance of the load resistor exceeds the resistance of the detector, the device operates in the "constant current mode," since the current to the detector is essentially constant. In this mode, the change in conductivity of the detector is usually sensed by measuring the voltage across the detector.
If the resistance of the load resistor is less than the resistance of the detector, the detector is operating in the "constant voltage mode," since the voltage across the detector is essentially constant. The change in the conductivity of the detector is usually sensed by measuring the voltage across the resistor.
Of these two modes, the constant current mode finds wider use in detectors made from semiconductor materials having low resistivity.
Photodetectors, and particularly arrays of such detectors, have many applications. One application is in the detection of infrared radiation. Infrared sensitive photodetector arrays are used for various heat and object sensing applications.
One widely used infrared-sensitive photodetective material is mercury cadmium telluride, which consists of a mixture of cadmium telluride and mercury telluride. Cadmium telluride is a wide-gap semiconductor (E.sub.g =1.6 eV), and mercury telluride is a semi-metal having a "negative energy gap" of about minus 0.3 eV. The energy gap of the alloy varies monotonically with x, the mole fraction of cadmium telluride in the alloy, Hg.sub.1-x Cd.sub.x Te. By properly selecting x, it is possible to obtain mercury cadmium telluride detectors having a peak response at any of a wide range of infrared wavelengths.
Mercury cadmium telluride photodetector arrays are now made by mounting a mercury cadmium telluride crystal on substrates with an epoxy. The thickness of the mercury cadmium telluride is then reduced to about 10 microns by lapping, polishing and etching. The detectors are then delineated and isolated by masking, and then cutting or etching. Electrical leads are attached to opposite ends of each of the individual detector elements, or to one end of each and a common.
Other methods for making such photodetectors are disclosed in the '223 patent. None of these methods is known to produce a detector that operates close to the theoretical maximum detectivity for radiation wavelengths in the range of about 1 to about 25 microns or more, particularly for wavelengths in the range of about 10 to about 25 microns or more, and particularly where such radiation appears in low background levels such as those having less than about 10.sup.17 photons/cm.sup.2 -sec.